This invention relates to impregnated cathodes.
Impregnated cathodes hold out a successful prospect for their ability to maintain high electron emission for a long time. Apart from such favorable high electron emission, a problem of the impregnated cathodes in that these cathodes must operate at a relatively high temperature between about 1050.degree. C. and about 1200.degree. C. which could cause evaporation of an electron emissive material, namely Ba and could result in a reduction in the service life of a heater.
In order to solve the foregoing problem, an attempt has been made wherein an electron emissive surface is coated with a layer of a precious metal, such as Os. Although this attempt has successfully lowered the operation temperature by 150.degree. C. and more, it has not found to be entirely effective due to mutual diffusion between the coating metal layer and the body material of the cathode, and more particularly due to oxidation of the Os coating layer.
Instead of the Os coating or the like metal coating, an impregnated cathode disclosed in Japanese Patent Laid-open Publication No. 58-154131 comprises a body of a sintered material composed of Sc.sub.2 O.sub.3 or oxides including Sc, such as (Al, Sc).sub.2 O.sub.3 and a refractory metal, such as W, and an electron emissive material impregnated in the sintered body. This cathode has an ability to operate at a lower temperature operation than that of the Os coated cathode. This cathode, so called Sc.sub.2 O.sub.3 mixed matrix body impregnated cathode, is characterized by an electron emissive surface coated with a mono-atomic layer of low work function elements, such as Ba, Sc and O. This layer is, however, defective in its service life because regeneration of the layer, which has been lost by some reasons, requires a long, high temperature heat treatment, and sometimes such regeneration becomes impossible.